Balanced positioning system for use in lithographic apparatus

ABSTRACT

A balanced positioning system for use in lithographic apparatus having a pair of balance masses which are supported so as to be moveable in at least one degree of freedom, such as Y translation. Oppositely directed drive forces in this degree of freedom act directly between the driven body and the balance masses to rotate the driven body about an axis perpendicular to the one direction. Reaction forces arising from positioning movements result in linear movements of the balance masses and all reaction forces are kept within the balanced positioning system.

This is a continuation application of U.S. application Ser. No.09/739,098, filed Dec. 19, 2000 now U.S. Pat. No. 6,449,030, the entirecontents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to balanced positioning systems. Moreparticularly, the invention relates to such systems in lithographicprojection apparatus comprising:

a radiation system for supplying a projection beam of radiation;

a first object table for holding a mask;

a second object table for holding a substrate; and

a projection system for imaging an irradiated portion of the mask onto atarget portion of the substrate.

2. Discussion of Related Art

For the sake of simplicity, the projection system may hereinafter bereferred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, catadioptric systems,and charged particle optics, for example. The radiation system may alsoinclude elements operating according to any of these principles fordirecting, shaping or controlling the projection beam of radiation, andsuch elements may also be referred to below, collectively or singularly,as a “lens”. In addition, the first and second object tables may bereferred to as the “mask table” and the “substrate table”, respectively.Further, the lithographic apparatus may be of a type having two or moremask tables and/or two or more substrate tables. In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more stages while one ormore other stages are being used for exposures. Twin stage lithographicapparatus are described in International Patent Applications WO 98/28665and WO 98/40791, for example.

Lithographic projection apparatus can be used, for example, in themanufacture of integrated circuits (ICs). In such a case, the mask(reticle) may contain a circuit pattern corresponding to an individuallayer of the IC, and this pattern can be imaged onto a target portion(comprising one or more dies) on a substrate (silicon wafer) which hasbeen coated with a layer of photosensitive material (resist). Ingeneral, a single wafer will contain a whole network of adjacent targetportions which are successively irradiated via the mask, one at a time.In one type of lithographic projection apparatus, each target portion isirradiated by exposing the entire mask pattern onto the target portionat once; such an apparatus is commonly referred to as a wafer stepper.In an alternative apparatus—which is commonly referred to as astep-and-scan apparatus—each target portion is irradiated byprogressively scanning the mask pattern under the projection beam in agiven reference direction (the “scanning” direction) while synchronouslyscanning the substrate table parallel or anti-parallel to thisdirection; since, in general, the projection system will have amagnification factor M (generally<1), the speed V at which the substratetable is scanned will be a factor M times that at which the mask tableis scanned. More information with regard to lithographic devices as heredescribed can be gleaned from International Patent Application WO97/33205.

In a lithographic apparatus, reactions on the machine frame toacceleration forces used to position the mask (reticle) and substrate(wafer) to nanometer accuracies are a major cause of vibration,impairing the accuracy of the apparatus. To minimise the effects ofvibrations, it is possible to provide an isolated metrology frame onwhich all position sensing devices are mounted, and to channel allreaction forces to a so-called force or reaction frame that is separatedfrom the remainder of the apparatus.

U.S. Pat. No. 5,208,497 describes a system in which the reaction of thedriving force is channeled to a balance mass which is normally heavierthan the driven mass and which is free to move relative to the remainderof the apparatus. The reaction force is spent in accelerating thebalance mass and does not significantly affect the remainder of theapparatus. However, the concept disclosed in U.S. Pat. No. 5,208,497 isonly effective for reaction forces in one direction and is not readilyextendable to systems having multiple degrees of freedom. Balance massesmoveable in three degrees of freedom in a plane are described in WO98/40791 and WO 98/28665 (mentioned above).

EP-A-0,557,100 describes a system which relies on actively driving twomasses in opposite directions so that the reaction forces are equal andopposite and so cancel out. The system described operates in twodimensions but the active positioning of the balance mass necessitates asecond positioning system of equal quality and capability to thatdriving the primary object.

None of the above systems is particularly effective at counteractingyawing moments which may be induced by adjustments of the rotationalposition of the driven mass or because of misalignment between the lineof action of forces exerted on the driven body and its center of mass.

U.S. Pat. No. 5,815,246 discloses a positioning system having a firstbalance mass free to move in an XY plane, i.e. to translate in X and Yand rotate about axes parallel to the Z direction. To control rotationof the first balance mass, a fly wheel, forming a second balance mass,is driven by a rotation motor mounted on the first balance mass to exerta counter-acting torque. Controlling rotation of the first balance masstherefore requires accurate control of the rotation and the flywheel.Any delay in this control or imbalance of the flywheel will causevibration.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved balancedpositioning system for counteracting yawing moments in the driven massand preferably also force balancing in at least two translationaldegrees of freedom.

According to the present invention there is provided a lithographicprojection apparatus comprising:

a radiation system for supplying a projection beam of radiation;

a first object table for holding a mask;

a second object table for holding a substrate;

a projection system for imaging irradiated portions of the mask ontotarget portions of the substrate; characterized by:

a balanced positioning system for positioning at least one of saidobject tables and comprising:

first and second balance masses;

bearing means for supporting said first and second balance masses so asto be substantially free to translate in at least one direction; and

driving means for acting directly between said one object table and saidfirst and second balance masses to rotate said object table about anaxis perpendicular to said one direction, said driving means beingarranged to exert linear forces on said first and second balance massesin opposite directions to effect said rotation of said object table.

By providing two balance masses that can translate in at least onedirection, the torque required to drive the object table to adjust itsrotational position, or to compensate for torques induced by otherdriving forces can be provided as the sum of two linearly acting forcesacting between the object table and the two balance masses. The reactionforces on the two balance masses will cause them to move linearly, whichcan easily be accommodated. In other words, the reaction to a torqueexerted on the driven object table is converted to translations of thetwo balance masses and no rotational movement of the balance massoccurs. It will be appreciated that if a rotational motion of the objecttable is combined with a linear motion, the net forces acting on eachbalance mass may be in the same direction, though different inmagnitude.

According to a yet further aspect of the invention there is provided amethod of manufacturing a device using a lithographic projectionapparatus comprising:

a radiation system for supplying a projection beam of radiation;

a first object table for holding a mask;

a second object table for holding a substrate; and

a projection system for imaging irradiated portions of the mask ontotarget portions of the substrate; the method comprising the steps of:

providing a mask bearing a pattern to said first object table;

providing a substrate provided with a radiation-sensitive layer to saidsecond object table;

irradiating portions of the mask and imaging said irradiated portions ofthe mask onto said target portions of said substrate; characterized inthat:

at least one of said object tables is positioned using a positioningsystem which includes first and second balance masses free to move in atleast one direction and drive means acting between said one object tableand said balance masses; and

during or prior to said irradiating step said one object table isrotated by exerting oppositely directed forces between it and said firstand second balance masses.

In a manufacturing process using a lithographic projection apparatusaccording to the invention a pattern in a mask is imaged onto asubstrate which is at least partially covered by a layer ofenergy-sensitive material (resist). Prior to this imaging step, thesubstrate may undergo various procedures, such as priming, resistcoating and a soft bake. After exposure, the substrate may be subjectedto other procedures, such as a post-exposure bake (PEB), development, ahard bake and measurement/inspection of the imaged features. This arrayof procedures is used as a basis to pattern an individual layer of adevice, e.g. an IC. Such a patterned layer may then undergo variousprocesses such as etching, ion-implantation (doping), metallisation,oxidation, chemo-mechanical polishing, etc., all intended to finish offan individual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4.

Although specific reference may be made in this text to the use of theapparatus according to the invention in the manufacture of ICs, itshould be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “targetportion”, respectively.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation or particle flux,including, but not limited to, ultraviolet radiation (e.g. at awavelength of 365 nm, 248 nm, 193 nm, 157 nm or 126 nm), EUV, X-rays,electrons and ions.

Embodiments of the present invention are described below makingreference to a Cartesian coordinate system with axes denoted X, Y and Zin which the XY plane is parallel to the nominal substrate and reticlesurfaces. The notation Ri is used to denote rotation about an axisparallel to the I direction.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described below with reference toexemplary embodiments and the accompanying schematic drawings, in which:

FIG. 1 depicts a lithographic projection apparatus according to a firstembodiment of the invention;

FIG. 2 is a plan view of the reticle stage of the apparatus of FIG. 1;

FIG. 3 is an end view of the reticle stage of the apparatus of FIG. 1;

FIG. 4 is a diagram of a servo control mechanism used in the firstembodiment of the present invention;

FIG. 5 is a plan view of the reticle stage of a second embodiment of theinvention;

FIG. 6 is an end view of the reticle stage of the second embodiment ofthe invention;

FIG. 7 is a plan view of the reticle stage of a third embodiment of theinvention;

FIG. 8 is an end view of the reticle stage of the third embodiment ofthe invention; and

FIGS. 9 and 9A show a cable ducting device useable in embodiments of theinvention.

In the drawings, like reference numerals indicate like parts.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiment 1

FIG. 1 schematically depicts a lithographic projection apparatusaccording to the invention. The apparatus comprises:

a radiation system LA, IL for supplying a projection beam PB ofradiation (e.g. UV or EUV radiation, x-rays, electrons or ions);

a first object table (mask table) MT for holding a mask MA (e.g. areticle), and connected to first positioning means for accuratelypositioning the mask with respect to item PL;

a second object table (substrate table) WT for holding a substrate W(e.g. a resist-coated silicon wafer), and connected to secondpositioning means for accurately positioning the substrate with respectto item PL;

a projection system (“lens”) PL (e.g. a refractive or catadioptricsystem, a mirror group or an array of field deflectors) for imaging anirradiated portion of the mask MA onto a target portion C (comprisingone or more dies) of the substrate W. As here depicted, the apparatus isof a transmissive type (i.e. has a transmissive mask). However, ingeneral, it may also be of a reflective type, for example.

The radiation system comprises a source LA (e.g. a Hg lamp, excimerlaser, an undulator provided around the path of an electron beam in astorage ring or synchrotron, or an electron or ion beam source) whichproduces a beam of radiation. This beam is caused to traverse variousoptical components comprised in the illumination system IL, —e.g. beamshaping optics Ex, an integrator IN and a condenser CO—so that theresultant beam PB has a desired shape and intensity throughout itscross-section.

The beam PB subsequently intercepts the mask MA which held on a masktable MT. Having traversed the mask MA, the beam PB is caused totraverse the lens PL, which focuses the beam PB onto a target portion Cof the substrate W. With the aid of the interferometric displacementmeasuring means IF, the substrate table WT can be moved accurately, e.g.so as to position different target portions C in the path of the beamPB. Similarly, the first positioning means can be used to accuratelyposition the mask MA with respect to the path of the beam PB, e.g. aftermechanical retrieval of the mask MA from a mask library. The referencesigns M1, M2 correspond to reticle alignment marks and the references P1and P2 correspond to wafer alignment marks. These marks are used toalign the wafer and the reticle respective to each other. In general,movement of the object tables MT, WT can be realized with the aid of along stroke module (coarse positioning) and a short stroke module (finepositioning), which are not explicitly depicted in FIG. 1.

The depicted apparatus can be used in two different modes:

1. In step mode, the mask table MT is kept essentially stationary, andan entire mask image is projected at once (i.e. a single “flash”) onto atarget portion C. The substrate table WT is then shifted in the x and/ory directions so that a different target portion C can be irradiated bythe beam PB;

2. In scan mode, essentially the same scenario applies, except that agiven target portion C is not exposed in a single “flash”. Instead, themask table MT is movable in a given reference direction (the so-called“scan direction”, e.g. the Y direction) with a speed v, so that theprojection beam PB is caused to scan over a mask image; concurrently,the substrate table WT is simultaneously moved in the same or oppositedirection at a speed V=Mv, in which M is the magnification of the lensPL (typically, M=¼ or ⅕). In this manner, a relatively large targetportion C can be exposed, without having to compromise on resolution.

FIGS. 2 and 3 show the reticle (mask) stage of the first embodiment ofthe invention in greater detail. The mask MA (not shown in FIG. 2),whose pattern is to be imaged onto the wafer, is held on mask table MT.To accommodate the scan mode of the apparatus the mask must bepositioned accurately over a relatively wide range of movement (stroke)in the Y direction but only over much smaller ranges of movement in theother degrees of freedom. This large Y-direction stroke, as well as amore limited stroke in the X-direction and some Rz movement, is effectedby the long stroke (coarse positioning) module described below. Finepositioning in all six degrees of freedom is accomplished by shortstrokeposition actuators included in the mask table.

Mask table MT depicted in FIGS. 2 and 3 is intended for use withtransmissive masks which means that the space above and below it must bekept clear. Accordingly, mask table MT is supported from two balancemasses 20, 30 positioned either side of a clear space extending in theY-direction. In the present embodiment, three beams 11, 12, 13, whichextend transversely from mask table MT, are provided for this purposebut the beams may alternatively be formed integrally with the body ofmask table MT or the mask table may itself extend over the balancemasses 20, 30. Balance masses 20, 30 have parallel planar upper surfacesagainst which table bearings 14, 15, 16 provided on the ends of beams11, 12, 13 act to support the mask table. Table bearings 14, 15, 16allow mask table MT to move in the XY plane relative to balance masses20, 30 substantially without friction. Table bearings 14, 15, 16 may,for example, be gas bearings. Z-direction actuators may also be includedin these bearings for coarse positioning in Z, Rx and Ry.

Balance masses 20, 30 are supported by substantially frictionlessZ-bearings 21, 22, 23, 31, 32, 33 on parallel rails 40, 50 which extendin the Y-direction and may be part of or connected to the main machineframe, or base plate, BP. Rails 40, 50 have substantially flathorizontal upper surfaces 41, 51 against which Z-bearings 21, 22, 23,31, 32, 33 act so that the balance masses 20, 30 are free to move in theY-direction over a relatively wide range of motion. Z-bearings 21, 22,23, 31, 32, 33 may be compliant, i.e. have a low stiffness, in theZ-direction so that the balance masses 20, 30 are also substantiallyfree to move in the Z-direction, though over a much smaller range ofmovement. Freedom for the balance masses to move in the X-direction maybe similarly provided by compliant X-bearings 24, 25, 34, 35 actingagainst substantially planar vertical walls 42, 52 of the rails 40, 50.X-bearings 24, 25, 34, 25 may be preloaded or opposed pad bearings toexert forces in both directions. Z-bearings 21, 22, 23, 31, 32, 33 andX-bearings 24, 25, 34, 35 may be, for example, gas bearings. The balancemasses 20, 30 are thus free to move in all three translational degreesof freedom and so provide balancing to the mask table in thosedirections. Rotational balancing in Rx and Ry is provided because theZ-bearings 21, 22, 23, 31, 32, 33 can be moved independently and arespaced apart. Balancing for Ry movements is provided by differentiallydriving the two balance masses 20, 30, as is discussed below.

If the ranges of movement of the mask table in the degrees of freedomother than Y translation are small, as is the case in the presentembodiment, the necessary freedom of movement of the balance masses canalso be accommodated by leaf spring arrangements, compliant bearings orother stiff bearings in combination with a gravity compensator. It isalso possible to arrange that reaction forces in some or all of theother degrees of freedom are only transmitted to one of the balancemasses so that only that balance mass needs to be supported withcontrolled compliance in the relevant degrees of freedom.

The stiffness of the bearings or supports in the other degrees offreedom and the mass of the balance mass(es) form a mass-spring systemthat acts as a low-pass filter, i.e. only low frequency forces aretransmitted to the machine frame. Significant attenuation of thereaction forces can be obtained if the natural frequency of thismass-spring system is substantially, for example 5 to 50 times, lowerthan the fundamental frequency of the actuation forces.

As will be described below, the mask table MT is driven by actuatorsacting against the balance masses 20, 30 so that they accelerate in theopposite direction to the mask table MT. The magnitudes of theaccelerations of the balance masses and the mask table MT will beproportional to their masses and so the ranges of movement of thebalance masses and the mask table in the various directions must be inthe ratio of their masses. To reduce the ranges of movement that must beprovided for the balance masses 20, 30 to accommodate the desired rangesof movement of the mask table MT, the balance masses 20, 30 are maderelatively massive, e.g. each 2 to 10 times the mass of the mask tableMT. The centers of mass of the balancing masses 20, 30 and mask table MTare preferably as close as possible in the Z-direction, e.g.substantially less than 100 mm, in order to minimise pitching or rollingmoments.

In the present embodiment, the mask table MT is driven in theY-direction by Y1-drive 18 acting between it and balance mass 20 andY2-drive 17 acting between it and balance mass 30. Y1- and Y2-drives 17,18 may, for example, comprise linear motors with an armature mounted tothe mask table MT and an elongate stator mounted to the respectivebalance mass. Yi-drive exerts, in operation, a force F_(yi) on the masktable MT and an equal and opposite reaction force R_(yi) on therespective balance mass.

Positioning in the X-direction is effected by a single X-actuator 19acting against balance mass 30. X-actuator 19 may also be a linear motorwith armature mounted to the mask table and stator mounted to thebalance mass or may be an elongated voice-coil motor free to displace inthe Y-direction, or a cylindrical voice-coil motor coupled to anaerostatic bearing that bears against a surface parallel to the YZplane. To enable the mask table to be driven in the X-direction whateverthe relative Y position of the mask table MT and balance mass 30, ifX-actuator is a linear motor, the stator must extend the whole of thecombined range of movement of the balance mass and mask table in the Ydirection. The line of action of the X-actuator 19 is preferablyarranged to pass through at least the Y-position of the center ofgravity CG_(MT) of the mask table MT so as to minimise the generation ofRz moments.

It follows from Newton's laws that if there is no rotational movement ofthe mask table, the displacements Δy_(b1), Δy_(b2) and Δy_(MT) of thebalance masses 20, 30 and mask table MT satisfy the followingconditions: $\begin{matrix}{{\frac{\Delta \quad y_{MT}}{\Delta \quad y_{b1}} = {{- \frac{m_{b1}}{m_{MT}}} \cdot \frac{l_{1} + l_{2}}{l_{2}}}};{\frac{\Delta \quad y_{MT}}{\Delta \quad y_{b2}} = {{- \frac{m_{b2}}{m_{MT}}} \cdot \frac{l_{1} + l_{2}}{l_{2}}}}} & \lbrack 1\rbrack\end{matrix}$

where:

l₁ and l₂ are respectively the distances in the X-direction between thecenters of gravity CG_(B1), CG_(B2) of the balance masses 20, 30 and thecenter of gravity CG_(MT) of the mask table MT; and

m_(b1), m_(b2) and m_(MT) are the masses of the balance masses 20, 30and mask table MT.

If m_(b1)=m_(b2)=m_(b) and l₁=l₂, then equation 1 can be reduced to:$\begin{matrix}{{\Delta \quad y_{b1}} = {{\Delta \quad y_{b2}} = {{- \Delta}\quad {y_{MT} \cdot \frac{m_{MT}}{2m_{b}}}}}} & \lbrack 2\rbrack\end{matrix}$

To effect a yawing (Rz) movement of the mask stage whilst stillcontaining the reaction forces within the balance mass system, theforces applied by Y1- and Y2-drives 17, 18 are controlled to takeadvantage of D'Alambert forces by moving the balance masses in oppositedirections. Note that if the yawing motion is effected at the same timeas a movement in Y, the balance masses may move in the same directionbut by differing amounts, thus the movement in opposite directions isrelative rather than absolute. For a counter-clockwise movement of themask stage by an angle θ_(MT) the necessary relative movements of thebalance masses are given by: $\begin{matrix}{{{\Delta \quad y_{b1}} = {- \frac{J_{MT} \cdot \theta_{MT}}{\left( {l_{1} + l_{2}} \right) \cdot m_{1}}}};{{\Delta \quad y_{b2}} = {- \frac{J_{MT} \cdot \theta_{MT}}{\left( {l_{1} + l_{2}} \right) \cdot m_{2}}}}} & \lbrack 3\rbrack\end{matrix}$

where J_(MT) is the moment of inertia of the mask table MT.

It should be noted that the present invention does not require themasses of the first and second balance masses to be equal nor that theybe disposed equidistantly about the centre of gravity of the mask table.

In a perfect, closed system, the combined center of mass of the masktable MT and balance masses 20, 30 will be stationary, however it ispreferable to provide a negative feedback servo system to correctlong-term cumulative translations (drift) of the balance masses thatmight arise from such factors as: cabling to the mask table and drives,misalignment of the drives, minute friction in the bearings, theapparatus not being perfectly horizontal, etc. As an alternative to theactive drift control system described below, a passive system, e.g.based on low-stiffness springs, may be used.

FIG. 4 shows the control loop of the servo system 130. The Y and Rzsetpoints of the balance masses with respect to the machine frame aresupplied to the positive input of subtractor 131, whose output is passedto the servo controller 132. Feedback to the negative input ofsubtractor 131 is provided by one or more multiple-degree-of-freedommeasurement systems 134 which measure the positions of the balancemasses and driven mass (mask table). The servo controller controls atwo-degree-of-freedom actuator system 133 which applies the necessarycorrections to the balance masses 20, 30. The positions of both balancemasses and driven mass may be measured relative to a fixed frame ofreference. Alternatively, the position of one, e.g the balance masses,may be measured relative to the reference frame and the position of thedriven mass measured relative to the balance masses. In the latter casethe relative position data can be transformed to absolute position dataeither in software or by hardware. Particularly in the Y-direction, theposition measurement may be performed by a linear encoder with a hightolerance to residual relative movements in the other degrees offreedom, such as those described in U.S. Pat. No. 5,646,730, forexample.

The set points of the servo system 130 are determined so as to ensurethat the combined center of mass of the mask table MT and balance masses20, 30 remains unchanged in the X, Y, Rz plane. This defines thecondition: $\begin{matrix}{{{m_{MT} \cdot {{\overset{\rightarrow}{u}}_{MT}(t)}} + {m_{b1} \cdot {{\overset{\rightarrow}{u}}_{b1}(t)}} + {m_{b2} \cdot {{\overset{\rightarrow}{u}}_{b2}(t)}}} = {{m_{MT} \cdot {{\overset{\rightarrow}{u}}_{MT}(0)}} + {m_{b1} \cdot {{\overset{\rightarrow}{u}}_{b1}(0)}} + {m_{b2} \cdot {{\overset{\rightarrow}{u}}_{b2}(0)}}}} & \lbrack 4\rbrack\end{matrix}$

where {right arrow over (u)}_(i)(t) is the vector position of mass i inthe X-Y plane at time t relative to a fixed reference point. The errorsignal between the calculated (using equation [4]) and measuredpositions is provided to the actuation system 133 which appliesappropriate correction forces to the balance masses 20, 30. The lowestresonance mode of the balancing frame and/or machine base is preferablyat least a factor of five higher than the servo bandwidth of the driftcontrol system.

The above described servo system can be used in the Y-direction onlywith drift control in the other degrees of freedom being performed bythe low stiffness of the supports for the balance masses in thosedegrees of freedom.

Embodiment 2

A second embodiment of the invention is shown in FIGS. 5 and 6 and isessentially the same as the first embodiment except as noted below.

The second embodiment is particularly applicable to lithographicapparatus employing reflective masks so that the space underneath themask table MT does not need to be kept clear. Advantage is taken of thisfact to support the mask table MT over a third balance mass 60. Thirdbalance mass 60 has a planar, horizontal upper surface over which isguided the mask table MT supported by bearings 71, 72, 73. Thesebearings may be, for example, gas bearings. Third balance mass 60 is inturn supported over the machine base frame by compliant bearings 61, 62,63, which may comprise low stiffness springs. The third balance massdoes not move in the XY plane so can alternatively be supported by leafsprings or gas cylinders without actual bearings. As illustrated, thesecond embodiment uses cylindrical voice coils 74, 75 in combinationwith X-bearings 76, 77 acting against the side of the second balancemass 30 for X-direction actuation. The X-bearings 76, 77 may be opposedpad bearings or preloaded so that forces in both directions can beexerted.

Embodiment 3

In a third embodiment, shown in FIGS. 7 and 8 and which is the same asthe first embodiment save as described below, the longstroke modulepositions a short, stroke frame 80 in Y and Rz only. Mask table MT isdriven relative to the short stroke frame 80 to position the mask in sixdegrees of freedom to a high precision. Such positioning is effected byshort stroke Z-actuators 81, 82, 83, X-actuator 84 and Y-actuators 85,86. The short stroke frame 80 is supported over first and second balancemasses 20, 30 by stiff Z-bearings 14′, 15′ 16′, which may be gasbearings acting on the planar upper surface of the balance masses. Shortstroke frame 80 is also constrained in X by bearing 78 relative to onlyone of the balance masses, in this case the second balance mass 30.

In the Y and Rz directions, the mask table MT moves with the short,stroke frame 80 so that in equations 2 and 3 the mass and moment ofinertia, m_(MT) and J_(MT), should be replaced by the combined mass andmoment of inertia of the mask table MT and short stroke frame 80.However, in the other degrees of freedom the short stroke frame 80 isconstrained to move with the balance mass and so increases the effectivebalancing mass, reducing its stroke. The center of gravity of the masktable MT is preferably coplanar, or close to coplanar, with that of theshort stroke frame 80 and balance masses 20, 30.

Embodiment 4

A cable ducting device according to a fourth embodiment of the inventionis shown in FIGS. 9 and 9A. Two cable ducts 151 a, 151 b are used tocarry cables and other conduits for utilities, such as control signalsand power, required by the mask table. The two cable ducts 151 a, 151 bare laid out in opposite directions between a terminal 152 mounted onthe mask table and a terminal 153 mounted on the machine frame so thatas the mask table moves in the Y direction, one cable duct is rolling upand the other is unrolling. The total length of cable duct moving withthe mask table therefore remains constant, whatever the Y position ofthe mask table. The moving mass therefore remains constant. Also, anyresidual tendencies of the cable ducts to roll up or unroll willcounteract each other. The cable ducts 151 a, 151 b have a slightlycurved cross-section, shown in FIG. 9A which is a cross-sectional viewalong the line A—A, in the same manner as a measuring tape. Thisprevents sagging and helps maintain a neat “U-shape” as the mask tablemoves.

Whilst we have described above specific embodiments of the invention itwill be appreciated that the invention may be practiced otherwise thandescribed. The description is not intended to limit the invention. Inparticular it will be appreciated that the invention may be used in thereticle or mask table of a lithographic apparatus and in any other typeof apparatus where fast and accurate positioning of an object in a planeis desirable.

Although this text has concentrated on lithographic apparatus andmethods whereby a mask is used to pattern the radiation beam enteringthe projection system, it should be noted that the invention presentedhere should be seen in the broader context of lithographic apparatus andmethods employing generic “patterning means” to pattern the saidradiation beam. The term “patterning means” as here employed refersbroadly to means that can be used to endow an incoming radiation beamwith a patterned cross-section, corresponding to a pattern that is to becreated in a target portion of the substrate; the term “light valve” hasalso been used in this context. Generally, the said pattern willcorrespond to a particular functional layer in a device being created inthe target portion, such as an integrated circuit or other device.Besides a mask on a mask table, such patterning means include thefollowing exemplary embodiments:

A programmable mirror array. An example of such a device is amatrix-addressable surface having a viscoelastic control layer and areflective surface. The basic principle behind such an apparatus is that(for example) addressed areas of the reflective surface reflect incidentlight as diffracted light, whereas unaddressed areas reflect incidentlight as undiffracted light. Using an appropriate filter, the saidundiffracted light can be filtered out of the reflected beam, leavingonly the diffracted light behind; in this manner, the beam becomespatterned according to the addressing pattern of the matrix-adressablesurface. The required matrix addressing can be performed using suitableelectronic means. More information on such mirror arrays can be gleaned,for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which areincorporated herein by reference.

A programmable LCD array. An example of such a construction is given inU.S. Pat. No. 5,229,872, which is incorporated herein by reference.

What is claimed is:
 1. A lithographic projection apparatus comprising: aradiation system which supplies a projection beam of radiation; a firstobject table to hold a mask; a second object table to hold a substrate;an imaging projection system which images irradiated portions of themask onto target portions of the substrate; a balanced object tablepositioning system which positions at least one of the object tables;first and second balance masses disposed along opposite sides of the atleast one of the object tables, the first and second balance massesbeing substantially free to translate in at least a first direction; anda pair of motors for moving the at least one of the object tables, eachmotor having two cooperating electromagnetic members, a first of themembers being mounted to the at least one of the object tables and asecond of the members being mounted to at least one of the balancemasses.
 2. The apparatus of claim 1, wherein the first and secondbalance masses are disposed along opposite sides of the first objecttable.
 3. The apparatus of claim 1, wherein the first and second balancemasses are disposed along opposite sides of the second object table. 4.The apparatus of claim 1, wherein the first and second balance masseshave substantially uniform cross sections orthogonal to the oppositesides.
 5. The apparatus of claim 1, wherein at least one beam extendsfrom the at least one of the object tables toward at least one of thebalance masses, the at least one beam being oriented substantiallyorthogonal to the first direction.
 6. The apparatus of claim 1, furthercomprising a drift control which limits drift of the balance masses. 7.The apparatus of claim 6, wherein the drift control comprises a servocontrol system and an actuator which applies forces to the balancemasses biasing the combined center of mass of the balance masses, theobject table positioning system, and the at least one of the objecttables to a desired position.
 8. The apparatus of claim 7, wherein thedrift control has a servo bandwidth at least a factor of five lower thana lowest resonance frequency of the balance masses and a base of theapparatus.
 9. The apparatus of claim 6, wherein the drift controlcomprises an active system.
 10. The apparatus of claim 6, wherein thedrift control comprises a negative-feedback servo system.
 11. Theapparatus of claim 6, wherein the drift control comprises a passivesystem.
 12. The apparatus of claim 6, wherein the drift controlcomprises at least one spring.
 13. The apparatus of claim 1, wherein themotors are adapted to translate the at least one of the object tables inthe first direction by exerting like-directed forces between the atleast one of the object tables and the first and second balance masses.14. The apparatus of claim 1, wherein at least one of the balance massesis also free to move in at least a second direction orthogonal to thefirst direction.
 15. The apparatus of claim 1, wherein the firstdirection is substantially parallel to the surface of a mask orsubstrate held on the second object table.
 16. The apparatus of claim 1,wherein a force applied by the first of the members onto the at leastone of the object tables results in equal and opposite reaction forcesbeing applied by the second of the members onto the at least one of thebalance masses.
 17. A lithographic projection apparatus comprising: aradiation system which supplies a projection beam of radiation; a masktable to hold a mask; a wafer table to hold a substrate; an imagingprojection system which images irradiated portions of the mask ontotarget portions of the substrate; a balanced object table positioningsystem which positions at least the mask table; first and second balancemasses independently disposed along opposite sides of at least the masktable, the first and second balance masses being substantially free totranslate in at least a first direction; at least one bearing supportingthe first and second balance masses; and a pair of motors for moving atleast the mask table, each motor having two cooperating electromagneticparts, a first of the parts being mounted to the mask table and a secondof the parts being mounted to at least one of the balance masses. 18.The apparatus of claim 17, wherein the first and second balance masseshave substantially uniform cross sections orthogonal to the oppositesides.
 19. The apparatus of claim 17, wherein at least one beam extendsfrom the mask table toward at least one of the balance masses, the atleast one beam being oriented substantially orthogonal to the firstdirection.
 20. The apparatus of claim 17, further comprising a driftcontrol which limits drift of the balance masses.
 21. The apparatus ofclaim 17, wherein at least one of the balance masses is also free tomove in at least a second direction orthogonal to the first direction.22. The apparatus of claim 17, wherein the first direction issubstantially parallel to the surface of a mask held on the mask table.23. A lithographic projection apparatus comprising: a radiation systemwhich supplies a projection beam of radiation; a first object table tohold a mask; a second object table to hold a substrate; an imagingprojection system which images irradiated portions of the mask ontotarget portions of the substrate; a balanced object table positioningsystem which positions at least one of the object tables; first andsecond balance masses disposed along opposite sides of the at least oneof the object tables and having substantially solid cross sectionsorthogonal to the opposite sides; a bearing supporting the first andsecond balance masses so as to be substantially free to translate in atleast a first direction; respective actuators, each of the respectiveactuators exerting forces on the at least one of the object tables andat least one of the balance masses; and a drift control which limitsdrift of the balance masses.
 24. The apparatus of claim 23, wherein thedrift control comprises a servo control system and an actuator whichapplies forces to the balance masses biasing the combined center of massof the balance masses, the object table positioning system and the atleast one of the object tables to a desired position.
 25. The apparatusof claim 24, wherein the drift control has a servo bandwidth at least afactor of five lower than a lowest resonance frequency of the balancemasses and a base of the apparatus.
 26. The apparatus of claim 23,wherein a linear actuator directly acts between the at least one of theobject tables and the first and second balance masses to rotate the atleast one of the object tables about an axis perpendicular to the firstdirection, the actuator being arranged to exert linear forces on thefirst and second balance masses in opposite directions to effect therotation of the at least one of the object tables.
 27. The apparatus ofclaim 26, wherein the linear actuator is adapted to translate the atleast one of the object tables in the first direction by exertinglike-directed forces between the at least one of the object tables andthe first and second balance masses.
 28. The apparatus of claim 26,wherein the at least one of the object tables is mounted on a frame andthe linear actuator acts between the frame and the first and secondbalance masses; the apparatus further comprising a short stroke actuatoracting between the at least one of the object tables and the frame forpositioning the at least one of the object tables in at least the seconddirection with a higher precision than a precision obtainable with thelinear actuator, wherein the frame is substantially rigidly coupled tothe one of the balance masses in the second direction.
 29. The apparatusof claim 26, wherein a second linear actuator between a frame and the atleast one of the balance masses is adapted to drive the at least one ofthe object tables in the second direction.
 30. The apparatus of claim23, wherein at least one of the balance masses is also free to move inat least a second direction orthogonal to the first direction.
 31. Theapparatus of claim 23, wherein the first direction is substantiallyparallel to the surface of a mask or substrate held on the second objecttable.